1. Field of the Invention
The present invention relates generally to power converters and, more specifically, the present invention relates to flyforward power converters.
2. Background Information
Power conversion circuits are typically designed to meet cost and efficiency targets defined at the start of a design. The type of power conversion circuit adopted in a particular design determines the overall system cost and operating performance.
One circuit configuration that provides the advantages of high efficiency and low system cost is a power conversion circuit called a flyforward converter. This circuit configuration effectively combines elements of two commonly used converter configurations, the flyback converter and the forward converter.
The flyforward converter includes individual flyback and forward transformers or energy transfer elements each having an input winding and at least one output winding. One of the advantages of the flyforward converter is circuit simplicity since only one power switch is required, which is coupled to the flyback and forward energy transfer element input windings across an input supply rail to the power conversion circuit. This power switch, switches on and off at a frequency determined by a control circuit coupled to the power switch. The frequency, which is the reciprocal of a switching cycle period, at which the power switch switches on and off may be fixed or variable depending on the type of control circuitry adopted.
The flyforward converter provides the combined advantages of efficient use of the flyback and forward energy transfer elements, low RMS current in the power switch and low ripple current in capacitors, which are coupled across the outputs of the flyback and forward energy transfer elements, as will be known to one skilled in the art.
However, the flyforward configuration suffers from a limitation in its operating characteristic, which restricts its use in many practical circuits. To describe this limitation, it is convenient to regard the flyforward converter in terms of the flyback energy transfer element and forward energy transfer element individually. In order for the forward energy transfer element to deliver energy to the power conversion circuit output, it is important that the magnetic flux in the forward energy transfer element at the end of a switching cycle period is reset to substantially the same value as it had at the beginning of the switching cycle period before the power switch is switched on.
During the following description the flux in the magnetic core of the forward energy transfer element at the beginning of a switching cycle period may be referred to as the initial value of the flux. In meeting this criterion, the magnetic core of the forward energy transfer element is prevented from saturating. In order for this operating criterion to be met, a reset voltage appears across the forward energy element input winding during the period of each switching cycle that the power switch is off.
To prevent the forward energy transfer element from saturating, the integral of this reset voltage during the power switch off time period is equal to the magnitude of the integral of the voltage appearing across the forward energy transfer element input winding during the power switch on period. This requirement is often referred to as the volt-second balance and ensures the magnetic flux does not build up in the magnetic core over a number of switching cycle periods, which would result in saturation of the magnetic core.
During the normal operation of a forward converter, in order to maintain the regulation of the voltage across the power conversion circuit output, the power switch on period as a percentage of the overall switching cycle period, which is referred to as the duty cycle, increases as the input voltage to the power converter decreases. The requirement to maintain the volt-second balance therefore requires that the magnitude of the reset voltage, integrated over the power switch off time, increases as the power converter input voltage decreases. This increased reset voltage increases voltage stress on the power switch as well as voltage stress on rectification diodes coupled to the output winding of the forward energy transfer element.
This typically limits the use of the flyforward converter to applications where the range of input voltage applied to the input of the power conversion circuit is very limited. This is a severe limitation since many applications having limited input voltage range specifications under normal operating conditions, have short term transient operating conditions where low input voltage must be tolerated with the power converter remaining fully operational.
Examples of applications where this could be a requirement are television and personal computer power conversion circuits. In these applications, if the input voltage to the power conversion circuit falls below the normal operating value, the power conversion circuit continues to operate long enough that memory back-up and other housekeeping functions can be completed by electronic circuitry coupled to the output of the power converter, before the power conversion circuit output voltage becomes too low. The period of time for which the power conversion circuit operates under these conditions is often referred to as the hold-up period. Although this is only a transient condition in the operation of the power converter, the limitations of the flyforward converter discussed above, make it necessary to rate the voltage of the power switch and output rectifiers to withstand this transient condition. This limitation can greatly increase the cost of the overall converter, making other converter topologies more attractive for this reason alone.